In the last several years many different approaches for automated detection of biological materials have been proposed and developed. These commonly are biosensors and biochip readers which often use live organisms or biological molecules, such as antibodies, nucleic acids (e.g., DNA chips), or enzymes as biological recognition elements to specifically bind target analytes. The specific binding of the target can be monitored by a recognition signal.
One of the most sensitive detection techniques available today is based on fluorescence excitation of dye-labeled targets. Current detection devices mostly fall into one of two categories, the first employs a white light source (usually a high power arc lamp) with CCD detector, and the second using laser excitation with photomultiplier tube (PMT) light collection in combination with a scanning technique. To meet the detection demands, a fluorescent scanner usually has a sensitivity of detecting at least 2-5 fluorphores per μm2; a resolution of 10 μm (pixel size) or better; and has a dynamic range of 5 orders of magnitude. Moreover, it needs to perform scanning of one slide in reasonable amount of time, typically five minutes or less per fluorescence channel. Problems with systems employing a white light source include the need of expensive filters and the short lifetime of arc lamp, which can be costly; while the approach using lasers is not practical for multicolor exaction due to the high cost of multiple lasers. Both types of scanners are costly and large enough to take up a substantial portion of a workbench.
A common disadvantage of all fluorescence based reading is the relatively inefficient use of the excitation light, due to the limited interaction with the fluorescence molecules. This increases the demand on the excitation source as well as on the detection system because unused excitation light is transmitted, scattered or absorbed elsewhere, decreasing the operational efficiency of the system and increasing background noise.
In order to improve the interaction, fluorescence readers employing optical waveguides have been proposed. A general disadvantage of conventional waveguide approaches is that the substance itself (e.g., liquid, which contains the molecules of interest) are not used as an optical waveguide, since the refractive index is lower than the index of the surrounding material (e.g., glass polymer, PDMS). Therefore, conventional optical waveguides typically provide only a weak interaction via evanescent waves with the target molecules, which are specifically bound to the waveguide surface. Existing waveguides also do not efficiently maintain light in the waveguide due to enhanced light scattering if the layer bound to the surface is inhomogeneous.
Thus, due to the required sensitivity of detection, and the inefficiency in the fluorescence excitation of existing systems, high powered light sources are necessary in order to obtain a sufficient amount of emitted fluorescing light. Such high powered light sources take up large amounts of physical space, require large amounts of energy to operate, and have a comparatively short life span requiring removal, replacement, and oftentimes realignment of lamp.
It is to be understood that fluorescent microscopes, as well as other detectors which employ fluorescence concepts, have the same issues regarding effective illumination and light collection from a sample and therefore face the same challenges as discussed above.